Photoconductive layer for recording element and method of producing same



I y 30, 1961 J. KOSTELEC ET AL 2,986,467

PHOTOCONDUCTIVE LAYER FOR RECORDING ELEMENT AND METHOD OF PRODUCING SAME Filed Dec. 17, 1958 INSULATING BINDER .PHOTOCONDUCTING MATERIAL 5 PHOTOCONDUCTIVE LAYER FLUOURSCENT MATERIAL 3 VINYL INTERPOLVMER MATRIX 2 FLUOURSCENT LAYER BASE INVEN7OR$ JOZE KOSTELEC HEINZ F. NITKA ATTORNEYS United States Patent PHOTOCONDUCTIVE LAYER FOR RECORDING ISELEMENT AND METHOD OF PRODUCING Joze Kostelec and Heinz F. Nitka, Binghamton, N.Y., assignors to General Aniline & Film Corporation, New York, N .Y., a corporation of Delaware Filed Dec. 17, 1958, Ser. No. 781,066

7 Claims. (Cl. 96-1) This invention relates to the general art of electrophotography and electroradiography and particularlyto a new and improved electrostatic recording element and a method of producing same. More specifically, the invention relates to electrophotographic members having a backing or support rendered electrically conductive by means of certain ionic compounds.

It is known to produce images or visible records by means of electrostatic printing methods. In such processes, a base plate or support is coated with a layer of photoconducting material which is then electrostatically charged in the dark. The charged layer is then exposed to light beneath a pattern such as a negative photographic film, positive film or a mask or other suitable light image whereupon a latent electrostatic image is formed upon the photoconductive surface. Such a latent image is produced by the neutralization of the electric charges in pro portion to the intensity of light to which any given area of the photoconductive layer is exposed. Development consists in dusting an electrically charged powder on the coating in the dark which adheres to the areas of high electrostatic charge (corresponding to low exposure) while the powder clings only slightly or not at all in the neutralized or discharged areas (corresponding to high exposure). The image can then be transferred to a suitable receiving material in order to obtain a positive or negative print as the case may be.

Other types of actinic radiations are also capable of producing electrostatic latent images on a charged surface of a photoconductor and in this connection mention is made of ultraviolet light, X-rays, gamma rays and the like. When electrophotographic plates are used in conjunction with X-rays, the process is known as electroradiography.

In electrophotographic processes generally described above, the recording element is commonly constructed in two different ways. In one arrangement, a layer or film of the photoconductive material is applied directly to a conductive metal backing member as exemplified by the type of construction known in the art as a xerographic plate. Alternately, the photoconductor may be employed in the form of minute particles dispersed in an electrically insulating binder and applied to a suitable backing member.

Whereas the xerographic plate requires a metal support or backing plate due to the type of construction employed, electrophotographic plates wherein the photoconductor is dispersed in an insulating binder can be coated on any convenient support. Of the two methods, the latter is preferred and offers advantages and conveniences over the xerographic plate, as is evident from the following comparison. The manufacture of xerographic plates is, at best, a costly and highly technical process. The metal plates must be specially treated in order to assure a scrupulously clean surface after which the selenium photoconducting layer is applied by an evaporation technique. The operation must be carried out under the most vigorously controlled conditions, particularly as regards to the rate of evaporation of the selenium and the even distribution and proper thickness of the selenium coating. The fact that the entire undertaking must be conducted under high vacuum greatly contributes to the cost of manufacture.

Furthermore, the use of xerographic plates is attendant with several disadvantages, chief among which is the need for making a transfer copy since the original electrophotographic recording element would be too bulky and expensive and generally unsuitable as a material for prints. Because the selenium photoconducting layer is very thin and also very soft, it easily becomes abraded or scratched during ordinary service and such defects are transferred to the final prints produced therefrom. In addition to the above disadvantages, such plates exhibit a fatigue effect after continued usage. This property or characteristic can be attributed to incomplete neutralization of the electrostatic charge on exposure to light resulting in a certain amount of permanent residual background charge. As a consequence, this residual charge attracts some of the developing powder giving rise to images and prints displaying darkened or foggy backgrounds. Although the plates may be rejuvenated by subjecting them to an electrostatic charge opposite in sign to the original charge so as to neutralize the residual background charge, this requires extra time and steps ing elements are easy to manufacture and do not require expensive and elaborate processing equipment. Furthermore, since the original recording material becomes the final print, all the disadvantages attendant to the use of intermediate plates are automatically eliminated.

, Up to the present time, however, electrophotographic recording plates wherein the support comprised a non-,

metallic backing were limited in their scope and application because of rather high background fog. This situation is due to the poor electrical conductivity of a nonmetallic support such as paper which retards conduction ofthe neutralized electrical charges away from the exposed areas of the plate. ground charge remains which attracts the developing powder to produce the foggy images previously mentioned.

It is believed to be manifest that the art is in need of an electrophotographic recording member capable of yielding fog-free prints directly which is, at the same time, economical to manufacture and which is easy to use and operate.

It is an object of this invention to provide an electrophotographic recording element wherein the photoconductive material is coated on a non-metallic conductive backing. A

Another object of the invention is to provide a method for coating the base of an electrophotographic recording element with a composition which will increase the elec-' a suitable recording material without having to. subse-.

quently transfer the first image to a receiving material.

Other objects and purposes of the invention apparent as the description proceeds.

Consequently, residual back- It has now been found that an efiicient and economical electrophotographic plate can be produced by treating a non-metallic base plate material such as paper with certain film-forming vinyl interpolymersof the type obtained from polymerizing a vinyl compound or copolyrneri'zing a vi'nyl compound with a product copolymerizable therewith in order to render said base plate relatively conducting after which a layer of photoconducting cornposition of the typewherein the photoconductor is dis persed in an electrically insulating binder is applied to the treated base plate.

Examples of such film-forming vinyl interpolyrners are described in copending application Serial Number 652,666, filed April 15, 1957, and include polyvinyl'acetate, copolymers of vinyl chloride and, vinyl acetate (86% of chloride to 14% of acetate), copolymers' of' vinyl chloride, vinyl acetate, vinyl alcohol (91% of chloride, 3% of acetate and 6% of vinyl alcohol), polyvinyl but yral, copolymers of vinyl butyral, vinyl alcohol and" vinyl acetate (84.5% of butyral, 13% of alcohol and 2.5 of acetate), hydrophilic copolymers of vinyl acetate with crotonic acid, acrylic acid and terpolyrnerssuch as vinyl acetate, acrylic acid, styrene and the like. A vinyl terpolymer which we have found particularly suitable for practicing the invention comprises an aqueous lower alcoholic alkaline solution containing a terpolymer obtained by polymerizing equimolar quantities of vinyl acetate, acrylic acid and styrene.

Typical backing materials whose conductivity can be the type wherein the photoconductor is suspended in the form of minute particles in an electrically insulating binding material having an electrical resistivity of about 10 to 10 ohms centimeter. The resulting electrophotographic plate can then be electrostatically charged" and developed in the manner commonly employed in the art. The so obtained prints are free of background fog and the use of a paper base in lieu of the usual metallic backing member results in the prints beingimmediately usable without resorting to any transfer processes.

The manner of preparing electrophotographic plates wherein the photoconductor is suspended or dispersed in an electrically insulating binder is described in the prior art and in this connection reference is made to U.S. Patent 2,663,636. Briefly, such a process comprises mixing and grinding together in a ball-mill or other suitable comminuting equipment a photoconducting mate rial together with a solvent and a binder material havingan electrical volume resistivity of about 10 to 10 ohms centimeter for a period of time sufiicient to eflect reduction to the desired particle size.

Our invention is particularly valuable when incorpoa rated in electrophotographic elements having a built-in intensifying screen of the'type described in U.S. QOPBIld? ing. application Serial Number 751,968, filed July 30, 1958; An electrophotographic element of the latter type comprises a paper support having coated thereon a fluorescent intensifying layer or screen which is then over.

coated by a second layer of a photoconductor dispersed inan' electrically insulatin binder. Ordinarily, such a multiple coating tends to yield images having a fairly high fluorescent layer. However, such a double layer coatipg can be made to yield images with little or nofbg-forming 4 background by dispersing particles of the fluorescent material in the above described film-forming vinyl interpolymers. Since the fluorescent particles are dispersed in a conducting environment, electrophotographic recording elements embodying this feature allow for easy neutralization and removal of the electric charges with the subsequent production of electrophotographic prints with,

fog-free background.

In the accompanying drawing, Figs. 1 and 2 represent sectional views of an electrophotographic member constructd in accordance-with our invention;

In Fig. 1, a baseplate 1 of paper has precoated' thereon a layer 2. of a film-forming vinyl interpolymer which, in

. turn, is overlayed with a photoconductiveinsulating layer 3 comprising an electrical insulating binder 4 having dispersed therein particles of a photoconducting material 5.

In Fig. 2, 1 is a base or backing support; 2 represents a fluorescent layer comprising a film-forming vinyl interpolymer matrix 3 having i-mbedded' or dispersed therein particles of a fluorescent material 4; and 5 is a photoconductive layer in which the photoconducting material 6 is dispersed or distributed throughout an electrically insulating'binder 7.

Examples of photoconductors which we can employ for the purpose described hereininclude zinc oxide, sulfur, anthracene, anthraquinone, lead iodide, cadmium sulfide, cadmium selenide and the like.

In some instances, it may be desirable to incorporate sensitizing dyes in electrostatic recording elements in order to alter the spectral response of'a particular photoconductive material. Thus, a sensitizing dye may be selected for the purpose' of increasing'the speed of the spectral response of a photoconductive material by extending or increasing the characteristic or inherent absorption of the photoconducting material itself. Or, a dye may be selected for the purpose of sensitizing the photoconducting material to a diiferent portion of the spectrum and thus extend the band of frequencies to which the photoconducting material will respond. Among the dyes which have been suggested as sensitizers for use in' electrophotography are the phthalein type dyes such as Rose Bengal, the triphenylmethane dyes such asmalachite green and methyl green, the cyanine dyes such as kryptocyanine, acridine orange, as well as many others.

It has been our observation that it is desirable to have the electrical conductivity of the backing plate or support of about the same value or higher as that of the photoconductor under the influence of the exciting radiation. However, regardless of the theory, the factrernains that a non-metallic base plate treated with the aforesaid film forming vinyl interpolymers results in an electrophotographic recording member which yields images having little or no fogged backgrounds.

Thefilm-formingvinyl interpolymer coating compositions areprepared by dissolving the-desired polymerin an appropriate solvent and the resulting mixture coated on a non-metallic base of the type described above. In some instances, it maybe desirable to add. various adjunctsto the composition in .order'to facilitate the coating operation. To this end, mention may be made of incorporating various dispersions of wax or wax-like materials, the function of which is to retard slippage by reducing the coefiicient of friction when such coatings are wound on magazines or spools. It may also be desirable to add spreadingagents to the coating compositions of the typedescribed wherein the purpose of which is toeffect'even distribution of the coating compositions on the base or support in order that the layers of'uniform thickness will ensue. v

In the interest of uniformity, all coatings weremade on paperand all the electrical measurements were carried;

out under identical conditions of temperature, humidity and illuniination.

Theinvention-is illustrated by the following examples.

It is to be understood, however, that no limitations are placed on the invention by such examples.

Example I A paper base was coated with a composition consisting of a terpolymer obtained by polymerizing equimolecular quantities of:

The desired alkalinity of the above solution is adjusted by the addition of alkali metal hydroxide solution until the pH ranges from 8.0 to 10.5 units with a preferred range of from 8 to 10 units. The resulting terpolymer coating was then allowed to dry. The resistivity of the paper base coated in such a manner was 4X10 ohms centimeter. This compares to 1.5x 10 ohms centimeter for the untreated paper.

A second coating consisting of a dispersion of a photoconductor in a binder was prepared as follows: 20 g. of zinc oxide, 16 g. of silicone resin and 20 g. of toluene were placed in a porcelain ball-mill of one quart capacity half filled with 0.5 in. porcelain balls and milled for about two hours. The mixture was then coated by applicator roller method on the above subbed paper base and allowed to dry. The thickness of the photoconductive layer was about 10 microns. An electrostatic charge was placed on the plate and exposed to radiation to effect discharge of the plate. The residual surface charge was then measured using a dynamic electrometer. A second electrophotographic plate was prepared as above except the paper base was not treated with the vinyl terpolymer. On charging and exposing, the residual surface charge on the second plate was much higher than in the case of the treated plate.

As a consequence of the low residual surface charge remaining on the electrophotographic member having the base treated as above, the resulting prints obtained therefrom are free of background fog.

The silicone resin was obtained on the commercial market as GE Silicone Resin SR-82, a product of the General Electric Company, Pittsfield, Mass.

The zinc oxide photoconductor was purchased commercially as French Process Florence Green Seal Pigment Grade, a product of the New Jersey Zinc Sales Company, Inc., New York, New York.

The vinyl terpolymer of vinyl acetate, styrene and acrylic acid as used above was prepared as follows: A mixture of 420 g. (1 molar equivalent) of distilled vinyl acetate, 702.8 g. (2 molar equivalents) of distilled acrylic acid and 504 g. (1 molar equivalent) of distilled styrene is copolymerized in ethanol solution in the presence of benzoyl peroxide for about 14-16 hours until the viscosity of the reaction mixture reaches a maximum comparable to that of glycerine. The mixture is cooled and poured into a solution of 590 g. of sodium hydroxide in 8 liters of water. 28 liters of water are added over a period of 25 minutes and the copolymer is precipitated by the addition of 6 N hydrochloric acid until a pH of 2 is reached. Upon acidification, the polymer forms a milky colloid which is effectively flocculated by passing steam through the colloid until a temperature of 58 C. is reached. The reaction mixture is then cooled to 35 C. by the addition of ice. The cooled polymer is filtered, washed, dried in air for one day and then dried in the oven until the odor of styrene has disappeared.

Fatty acids which can be combined with the triple polymer include:

Laurie acid Myristic acid Palmitic acid Stearic acid Oleic acid Example II The same procedure was followed as given in Example I excepting that the zinc oxide photoconductor was sensitized to the green portion of the spectrum with Rose Bengal dye. 0.01 gram of sensitizer was added to the zinc oxide dispersion during the ball-milling stage.

Example III The same procedure was followed as given in Example I excepting that the vinyl interpolymer used in this case was a 5% (by weight) aqueous solution of the sodium salt of the copolymer of methyl methacrylate-meth acrylic acid. The electrical resistivity of the resulting base coating was 8.2.)(10 ohms centimeter.

Example IV 15 grams of zinc sulfide (copper and cobalt activated) and 20 g. of the vinyl terpolymer solution of Example I was ball-milled for two hours. The resulting dispersion of fluorescent agent was then coated on a metal base using the applicator roller method. The thickness of the layer amounted to about 10 microns. A photoconductive layer of the type described in Example II was next applied over the first layer and allowed to dry. The resulting elect-rophotographic recording element containing an intermediate fluorescent layer interposed between the base and photoconductive layer is particularly valuable in the field of radiography wherein the exciting radiation are X-rays. In operation, the element is first electrostatically charged in the usual manner and then exposed to an X-ray pattern or image. At the exposed areas, the electrostatic charges comprising the latent image are neutralized. In addition to the direct discharge of the latent image, the X-rays cause the fluorescent layer to emit light which neutralizes more of the electrostatic charges. Thus, the combination of a fluorescent substance and photoconductor operate in a synergistic manner to increase the efiiciency of the system.

Example V The same procedure was followed as presented in Example IV excepting that the vinyl terpolymer was replaced by a 5% (by weight) aqueous solution of the sodium salt of the copolymer of methyl methacrylatemethacrylic acid.

Example VI The same procedure was employed as given in Example IV excepting that the metal base was replaced by a paper base having the vinyl terpolymer precoating of Example I.

Example VII The same procedure was followed as presented in Example IV excepting that the vinyl terpolymer was replaced by a 10% (by weight) aqueous solution of the sodium salt of the copolymer of methyl methacrylatemethacrylic acid.

We claim:

1. In an electrophotographic element comprising a non-metallic base plate having thereon a photoconductive insulating layer comprising a zinc oxide photoconductor uniformly dispersed in an electrically insulating binder, the electrical resistance of which is higher than the individual resistances of the base plate and photoconductor, the improvement which comprises interpolating between the base plate and photoconductive layer a stratum of a film-forming vinyl polymer selected from the class consisting of an interpolymer of a vinyl ester, acrylic acid and styrene, and a copolymer of methyl methacrylate and methacrylic acid.

2. In an electrophotographic element comprising a non-metallic base plate having thereon a photoconductive insulating layer comprising a zinc oxide photoconductor uniformly dispersed in an electrically insulating hinder, the electrical resistance of which is higher than the individual resistances of the base plate and photo- 7. conductor, the improvement which comprises interpolating between the base plate and photoconductive layer a stratumof an' interpolymer of a vinyl ester, acrylic acid and styrene.

3. In an electrophotographic element comprising a non-metallic base plate having thereon a photoconductive insulating layer comprising a zinc oxide photoconductor uniformly dispersecl'in an electrically insulating binder, the electrical. resistance of which is higher than the individual resistances of the base plate and photoconductor, the improvement which comprises interpolating between the base plate and photoconductive layer a stratum of a copolymer of methyl methacrylate and methacrylic acid.

4. The product according to claim 1 wherein the zinc oxide is dye sensitized.

5. The article as defined in claim 1 wherein the electrically insulating binder is selected from the class consisting of silicone resins, cellulose esters, cellulose ethers, vinyl resins, waxes and natural resins.

6. The artcle is defined in claim 1 wherein the electrically insulating binder is a silicone resin.

7 The article as defined in claim 1 wherein the base plate is paper.

7 References Citedin the file of this patent UNITED STATES PATENTS 2,774,921 Walkup Dec. 18, 1956 2,825,814 Walkup Mar. 4, 1958 2,844,734 l lantmann July 22, 1958. 2,885,556 Gundlach May 5, 1959 2,886,434 Owens May 12, 1959 v FOREIGN PATENTS 157,101 Australia Nov. 1, 1956 201,301 Australia Oct. 18, 1954 203,907 Australia Nov. 1, 1956 OTHER REFERENCES McMaster: Non-Destructive Testing, vol. 10, No. 1,

Young et a1.: R.C.A. Review, vol. XV, No. 4, pp. 469-484 (1954). 

1. IN AN ELECTROPHOTOGRAPHIC ELEMENT COMPRISING A NON-METALLIC BASE PLATE HAVING THEREON A PHOTOCONDUCTIVE INSULATING LAYER COMPRISING A ZINC OXIDE PHOTOCONBINDER, THE ELECTRICAL RESISTANCE OF WHICH IS HIGHER THAN BINDER, THE ELECTRICAL RESISTANCE OF WHICH IS HIGHER THAN THE INDIVIDUAL RESISTANCES OF THE BASE PLATE AND PHOTOCONDUCTOR THE IMPROVEMENT WHICH COMPRISES INTERPOLATING BETWEEN THE BASE PLATE AND PHOTOCONDUCTIVE LAYER A STRATUM OF A FILM-FORMING VINYL POLYMER SELECTED FROM THE CLASS CONSISTING OF AN INTERPOLYMER OF A VINYL ESTER, ACRYLIC ACID AND STYRENE, AND A COPOLYMER OF METHYL METHACRYLATE AND METHACRYLIC ACID. 